Slot type magnetron



Jan. 26, 1954 E. M. PURCELL SLOT TYPE MAGNETRON Filed Feb. 18, 1946FIG.|

FIG.4

M/VEA/TOR. EDWARD M. PURCELL Z/WAQM /-e/e.

ATTORNEY Patented Jan. 26, 1954 2,667,601 SLOT TYPE MAGNETRON Edward M.Purcell, Cambridge, Mass, assignor, s, to the United States of Americaas represented by the Secretary of by mesne assignment ApplicationFebruary 18, 1946, Serial No. 648,524 7 Claims. (01. 31539) Thisinvention relates in general to electronic oscillators and moreparticularly to oscillators of the cavity magnetron type.

Cavity magnetrons are a type of electronic oscillator finding mostwidespread use in the centimeter wavelength region. A magnetron of thistype is characterized by a relatively long, straight cathode having anumber of cavity resonators, all tuned to substantially the samefundamental frequency, positioned symmetrically about the oathode and ata short distance from it, and forming a portion of the anode structurethereby. These cavities have open ends which face the cathodeanode spaceand when a magnetic field is set up parallel to the cathode axis, and anelectric field is applied perpendicular to the cathode axis, electronswhich are emitted by the heated cathode will travel in spiral-like pathsin the cathodeanode space. As each electron passes the vicinity of theopening in a cavity, it induces an electromagnetic field therein and,due to the coupling between individual cavities, the electromagneticoscillations are generated in the correct phase relationships.

Since these cavities have dimensions of the order of magnitude of a wavelength at the operating frequency, it will be seen that tubes operatingin the centimeter wavelength region will have very smal1 cavities. It isfurther apparent that a small error in the dimensioning or forming ofthe cavities will cause a relatively large error as percentage of awavelength, and thus causing a, relatively large error in the resonantfrequency of that particular cavity. Thus cavities of these smalldimensions are difficult to form with the desired accuracy.

Before the conception of the present invention, most of the cavitymagnetrons of a practical nature had cavities of the shape shown in Fig.6 of the application of Prescott D. Crout entitled Electrical Apparatus,Serial No. 580,013, and filed February 27, 1945, now Patent No.2,639,403. It can be seen that this design of cavity would be verydifficult to manufacture with sufiicient accuracy in the lower region ofthe centimeter Wavelength range.

Accordingly, among the objects of the present invention are:

1. To provide a cavity magnetron for operation in the lower end of thecentimeter wavelength region;

2. To provide such a cavity magnetron having cavity resonators of such atype that they will be comparatively easy to construct; and

3. To provide such a. cavity magnetron having cavity resonators whichcan be manufactured with a reasonable accuracy in resonant fre-'-quency.

In accordance with the present invention there is provided a cavitymagnetron in which the individual cavities are rectangular in shape, asseen looking parallel to the cathode axis. At very small wave lengths,these cavities efiectively become slots cut in the anode block. Sincethese slots may be formed with a reasonable degree of accuracy bysawing, breaching, or other means, the anode structure will berelatively simple to manufacture. Other advantages will be apparent fromthe specification hereinafter provided.

This invention will best be understood erence to the drawings, in which:

Fig. 1 is a detailed drawing of a magnetron incorporating the principlesof the present invention, this view being in a plane perpendicular tothe cathode axis;

Fig. 2 is a detailed drawing of the magnetron of Fig. 1 in a planeparallel to the cathode axis;

Fig. 3 is a partial side view of an alternate type of output coupling tothat used in Fig. 1 and which may be used in connection with a magnetronincorporating the principles of this invention;

Fig. 4 is a top view of the type of coupling of Fig. 3; and

Fig. 5 shows the approximate variation of electric and magnetic fieldsalong the radial length of a cavity.

Referring now to a description of the magnetron shown in Figs. 1 and 2,which embodies the principles of the present invention, there is shown aview looking into the end space parallel to the straight cathode in thecase of Fig. 1, and a view along section AA in the case of Fig. 2. Theend been removed, and the source of magnetic field is not shown, for thesake of clarity. A tubular cathode 10, on the surface of at least aportion of the length of which is a coating of electronemittingmaterial, has a helically-shaped heater Wire I I positioned within thecathode l 0. At the ends of the cathode I 0 are plugs 12 and I 3.

by ref axial line the outer conductor of which is l9, while sleeve I! ispress-fitted to the center conductor 20 of a coaxial line the outerconductor space covers of this magnetron have as tantalum. Sleeve IB'is.press-fitted to the center conductor l8 of a coof which is 2i. Leads 18and 2B are connected to a source of alternating current, not shown,forheater power. Hats 22 and 23, of a material such as tantalum, are heldby leads 24 and 25 respectively, which are welded to the hats and tosleeves l6 and [1 respectively. These hats are positioned substantiallysymmetrically over .the ends of the cathode.

Arranged symmetrically about the cathode I is an anode structurecomprising ten trapezoidalshaped copper blocks 26, 27, 28, 29, 3B, 3!,32, 33,

' 34 and 35. Surrounding this anode stnuctnlsefiszan annular shell 36,which is usually held at ,ground potential. It is therefore:seenthat-the various copper blocks form the sides, the backs, of tencavity resonators S'l, 38, 39, '46, 4|, 42, 43, 44, 45 and 46. eachcavity, as shown in Fig. leis thus-seentobe rectangular in shape, and ofa comparatively narrow width. In practice the length of each of thesecavities along a radius is made to heapproximately oneuarter of awavelength at .the operating frequency of the magnetron. The shape of.acavity slot, along .an axis parallel to the axis voi the cathode I0, isshown in Fig.2 to be rectangular 141), with a depth several t mes t e d'o th sl t- 'The outer conductors 119 and '2] which shield the ,leadsbringing in the 'heaterpowerflhave their e ds th a ed so th t y maybescrewe i to thread d h 'i s ll .36. as sh wn onthe drawinesfI'he radiofrequency power generated by the mag etron is ex acted from the endspace directly above cavity 4'6by coupling loop 48. This loop is weldedto the end of .therinner conductor 49 and to the inside of the outerconductor 59 of an output coaxial -line. The outer con ctor 50isrthreaded, a d ad pted to screw into .a threaded Thole the shell 3.6.as shown on the drawings.

.Figs. ,3 and-A vshowan put couplin whi h a ama netmn inc rporatin the pin iples of the present invention. Fig.3 is a View showing .a sectionthrough one of the trapezoidal-shaped copper blocks-the plane of the,paper being parallel to the plane of the cathode. The face 61 of theblock 69 i adjacent to the cathode-anode space of the magnetron. In thetop QPP pin .62 in ,a Pub fit with the block. A copper wire .53 issoldered to this copper pin 52 and to this block 60, with silver solderbeing preferably used. This wire .63 may then become the centerconductor .of acoaxial line. It is noted that the connectionlbetweenlead .63 and block .50 takes place quite near to the edge .Bl .of theblock. Fi '5 shows the outline of a slot 1.0, the .plane of the paper inthis case being perpendicular .to the cathode axis. .Curve ll .shows theapproximate variatiQn of the magnetic field strength ,H 01 theoscillating energy .in the .cavity space at one particular instant oftime. In this graph, disstance is plotted hori ontally, corresponding tothe distance along the slot 1!}, and magnetic field strength H isplotted vertically. Curve I2 is a graph of the approximate variation ofthe electric field E :of'the oscillating energy in the .cavity space atone particular instant of time. In this graph, distance along the cavityis plotted hori mentall and electric field strength-E is plottedvertically.

alternative type -.of outbe satisfactorily used in Referring-nowtea-description of the operation of {the magnetron of Figs. :1 and -'2,which incorlandshll "36 :forms The cross-section of g of this blockGU-isasmaIl r porates the principles of the present invention, it isseen that the cavity resonators making up the tuned circuits of the tubeare of along, rectangular shape. Having the cavities of this shapeallows the maximum amount of heat conduction away from the cathode-anodespace through the anode structure, with respect to practical shapes "ofcavities, iTheoretically,-ithe amount of heat conduction, which isdependent to a large extent it .on the amount of cross-sectional area inthe anode blocks, such as 26, 21, etc., in a plane normalttaaidiametrical plane, may be increased by form- .ing the cavities in a.trapezoidal sha e, with the width of the .outer extremities of thecavity being Illessthan the midi 1120f the cavity at its inside end.

However, the properties of this type of cavity would be unsatisfactorycompared with the properties-of the-rectangular type of cavity accordingto-thezpresent invention. Thus the present shape of cavity allows formaximum heat conduction from the. anodestructure .andirom thecathodeanodespace using practical. shapes .of cavities, as compared tomagnetrons liavfmglthe same inner and .outeranode structure .diameterbut .idifierent shapes .of cavities. .A tube employing. aho'le-andslottype .of anode structuresuch as is shownintheabove-mentioned.application of Crout,.Fi g. I6, must ,be made .withthe .hole diameters .so'llarge with respect :to 'the .diameterrof thecircle. around which .the holes are placed that .the lateral .distancebetween holes, as .measured between the nearest points .on .theircircumference, ,forms a narrowconstrictiomand thereforea highimpedance,.tmtheconduction of heat.

.The main advantage, however, in constructin a magnetron with cavitiesof .the rectangular shape .lies in the ease of forming the cavities. Onemethod, for instance, .of forming the anode block is tosprovide acylindrical block of copper and to drill a hole-of a diameterequalto theinner diameter .of the anode .segments 26, :2 I, etc. .in the block..The slots .may then be formed by sawing the .anode bloclg or bybreaching. .Thus eachslot 31, 38, etc. is formed by one machiningoperation. A mag-netronpf the hole-cand slot typ.e, such as is shown :inFig. '6 of :the above-mentioned application of Brent .must take at least:two machining operations to dorm the cavities, and the dimensioning ofthe-pavitiesis more complex. {The relatively simple constructionprocedure of the slot type ranode structure according to this inventionthus makes the accurate manufacture :of very short wavelength magnetronsrelatively simple.

'The approximate distributions of electric and magnetic field-strengthsexisting :in the rectangular shape of cavity as used :in a cavitymagnetron are shown in Fig. .5. The cathode-anode space is to the leftof the rectangular cavity :19. This cavity maybe thought-ofas,ashort-circuit-termihated transmission line having a lengthapproximately equal to -:a quarter of a wavelength at the operatingfrequency of the magnetron. Thus ordinary transmissionline theorywilleexplain the approximate field distributions within the cavity. Themagnetic field strength H starts out :at the front of the cavity with avery low value. :It then increases gradually until a maximum value isreached at the back of the cavity, as shown by curve "H. The electricfield strength 'E, on the other hand, is maximum atth-e-front of thecavity and decreases gradually until the field strength is very low inthe electric field strength I; are substantially constant along thedepth of the cavity, i. e., along an axis parallelto the cathode axis'.Thisof course neglects the effects of the end spaces. f

As can be seen from electromagnetic field theory, the width of thecavity (its shortest dimension) determines the effective inputcapacitance of the cavity. Because these resonators are just smallslots, the effect input capacitance is relatively high. The effectiveinductance of the cavity is dependent upon the length of the cavity,along a radius, and the shape of the cavity. The input inductance of ahole-and-slot type cavity is larger than the inductance of the slot typecavity, because the magnetic field strength at the back of the formertype of cavity is the stronger of the two, other factors being equal.Thus the slot type cavity presents a relatively low impedance to thecathode-anode electromagnetic field, it having a relatively higheffective capacitance and a relatively low efiective inductance.

Another advantage of types of magnetrons according to the presentinvention over the holeand-slct type is that more cavities are able tobe positioned about a given diameter cathodeanode space in the formertypes.

Due to the narrow width dimension of this type of resonator, and becauseof the very short wavelengths involved, there is necessitated a type ofcoupling means external to the cavities themselves. Two such couplingarrangements are shown in Figs. 1 and 2 and Figs. 3 and 4.

The output coaxial line has its center conductor 49, Figs. 1 and 2,soldered to a small loop 48, which is bent around and welded to theouter conductor 50. The electromagnetic fields from each cavity arecoupled into the end space just above the resonators, and the magneticfield causes current flow in coupling loop 48, thus extracting energyfrom this space.

Hats 22 and 23, which are kept at the same potential as the cathode, areplaced directly over the cathode-anode space to repel any electronswhich may stray out into the end space, back to the cathode-anode space.

In the type of coupling shown in Figs. 3 and 4, a copper wire 63 issoldered to a point on the top of one of the copper blocks 60 making upthe anode structure such that the correct eiTective matching impedanceis provided. It is desirable to make this wire 63 the central conductorof a coaxial line, so that a minimum amount of radiation may occur fromthis output circuit.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

l. A cavity type magnetron comprising a cathode having a substantiallystraight axis, an anode structure, said anode structure comprising aplurality of substantially trapezoidal-shaped blocks positioned aboutsaid cathode, said metal- 2.- A cavity "type magnetron comprising acathode having a substantially straight axis, an anode structure, saidanode structure comprising a plurality of trapezoidal-shaped blockspositioned about said cathode, said metallic blocks forming a pluralityof cavity resonators, the cross section of each of said cavityresonators in a plane normal to the axis of the cathode beingsubstantially rectangular in shape, and means for coupling highfrequency energy from said magnetron comprising a coaxial line includingan inner conductor and an outer conductor, said outer conductor adaptedto be electrically connected to' a source of reference potential, theend of said inner conductor having the form of a loop and beingelectrically connected at its extremity to said outer conductor, theloop being positioned above said anode blocks.

3. A cavity type magnetron comprising a cathode having a substantiallystraight axis, an anode structure, said anode structure comprising aplurality of trapezoidal-shaped metallic blocks positioned about saidcathode, said metallic blocks forming a plurality of cavity resonators,the cross section of each of said cavity resonators in a plane normal tothe axis of the cathode being substantially rectangular in shape, andmeans for coupling high frequency energy from said magnetron comprisinga conducting means coupled to one of said metallic blocks.

4. A magnetron comprising a cathode, an anode, said anode comprising aplurality of metallic blocks positioned about said cathode, adjacentsides of said metallic blocks forming a plurality of cavity resonatorshaving parallel Walls extending away from said cathode, and means forcoupling high frequency energy from said magnetron comprising aconducting means coupled to one of said metallic blocks.

5. A magnetron as defined in claim 4, wherein the width of said slotsbetween said parallel walls is small relative to the other dimensions ofsaid slots.

6. A magnetron comprising a cathode having a substantially straightaxis, an anode comprising a metallic block having a cylindrical holecoaxial with said cathode and a plurality of wedge-shaped slotsextending radially from said cylindrical hole, the cross-section of eachof said slots in a plane normal to the axis of said cylindrical holebeing substantially rectangular, the width of said slots inthecircumferential direction being small relative to their radial and axialdimensions, said slots constituting the resonant cavities of said anode,and means for coupling high frequency energy from said magnetroncomprising a conducting wire, said wire being coupled to one of saidmetallic blocks.

7. A cavity type magnetron comprising a cathode having a substantiallystraight axis, and an anode having a plurality of resonant cavities,said anode having a cylindrical hole coaxial with said cathode, each ofsaid resonant cavities consisting of a slot extending radially from saidcylindrical hole, the cross-section of each of said slots in a planenormal to the axis of said cathode being substantially rectangular, thewidth of said slots in the circumferential direction being smallrelative to their radial and axial dimensions, and means for couplinghigh frequency energy from said magnetron comprising a coaxial lineincluding an inner and outer conductor, said outer conductor adapted tobe electrically connected to a source of reference potential and saidinner conduster-extending beyond he-outer cunductgr into N mbe thespace'immediately above the amzde. 2,111,151 EDWARD M. PURCELL.2,411,601 2,415,253 References Cited in the file of this patent 5 UNITEDSTATES PATENTS I Number Number Name Date 20 5 1,684,947 Daumann Sept.16, 1928 215,600 2,063,342 Samuel Dec. '8, 1936 2,408,234 Spencer Sept.24, 1946 10 8 4 Name 1 Da e 7 ?e-, r--"'-.-- NOV Sp ncer Nov. 26, 1946Lmde; Feb. 4, 1947 V FOREIGN PATENTS Country Date Switzerland 1 May 1,1940 Switzerland Oct. .16, 1941

